APPENDIX I. 457 



Chemical equilibrium in general, and dissociation in particular, are now 

 being so fully worked out in detail, and supplied in such various ways, that I 

 do not allude to them to develop, but only use them as examples by which 

 to indicate the correctness of a tendency to regard chemical combinations 

 from points of view differing from those expressed by the term hitherto ap- 

 propriated to define chemical forces, namely, ' affinity.' Chemical equilibria, 

 dissociation, the speed of chemical reactions, thermochemistry, spectroscopy, 

 and, more than all, the determination of the influence of masses and the 

 search for a connection between the properties and weights of atoms and 

 molecules in one word, the vast mass of the most important chemical re- 

 searches of the present day--clearly indicate the near approach of the time 

 when chemical doctrines will submit fully and completely to the doctrine 

 which was first announced, in the Principia of Newton. 



In order that the application of these principles may bear fruit it is evi- 

 dently insufficient? to assume that statical equilibrium reigns alone in chemical 

 systems or chemical molecules: it is necessary to grasp the conditions of 

 possible states of dynamical equilibria, and to apply to them kinetic prin- 

 ciples. Numerous considerations compel us to renounce the idea of statical 

 equilibrium in molecules, and the recent yet strongly- supported appeals to 

 dynamic principles constitute, in my opinion, the foundation of the modern 

 teaching relating to atomicity, or the valency of the elements, which usually 

 forms the basis of investigations into organic or carbon compounds. 



This teaching has led to brilliant explanations of very many chemical 

 relations and to cases of isomerism, or the difference in the properties of 

 substances having the same composition. It has been so fruitful in its many 

 applications and in the foreshadowing of remote consequences, especially 

 respecting carbon compounds, that it is impossible to deny its claims to be 

 ranked as a great achievement of chemical science. Its practical application 

 to the synthesis of many substances of the most complicated composition 

 entering into the structure of organised bodies, and to the creation of an un- 

 limited number of carbon compounds, among which the colours derived from 

 coal tar stand prominently forward, surpass the synthetical powers of Nature 

 itself. Yet this teaching, as applied to the structure of carbon compounds, 

 is not on the face of it directly applicable to the investigation of other ele- 

 ments, because in examining the first it is possible to assume that the atoms 

 of carbon have always a definite and equal number of affinities, whilst in the 

 combinations of other elements this is evidently inadmissible. Thus, for 

 example, an atom of carbon yields only one compound with four atoms of 

 hydrogen and one with four atoms of chlorine in the molecule, whilst the 

 atoms of chlorine and hydrogen unite only in the proportions of one to one. 

 Simplicity is here evident, and forms a point of departure from which it is 

 easy to move forward with firm and secure tread. Other elements are of a 

 different nature. Phosphorus unites with' three and with five atoms of 

 chlorine, and consequently the simplicity and sharpness of the application of 

 structural conceptions are lost. Sulphur unites only with two atoms of 

 hydrogen, but with oxygen it enters into higher orders of combination. The 

 periodic relationship which exists among all the properties of the elements- 

 such, for example,, as their ability to enter into various combinations and 



